1. Field of the Invention
The present invention relates to a printed wiring board, and more particularly to a printed wiring board excellent in reliability for surface mounting.
2. Description of the Related Art
Recently, as electronic appliances are becoming smaller in size and higher in density, the mounting method on the printed wiring board is mainly dominated by the so-called chip-on-board (COB) mounting system for mounting semiconductor parts directly on the printed wiring board.
In such surface mounting method, generally, the printing wiring board is manufactured by using a copper lined laminate (FR-4) for printed wiring board made of a glass cloth base material impregnated with an epoxy resin, and surface mount devices (SMDs) are mounted on the surface.
However, surface mount devices differ in size, shape and connection method, and also vary in the behavior (thermal expansion) due to temperature (heat). In particular, ceramic parts and bare chip parts are about 3 to 7 ppm in the coefficient of thermal expansion, and the parts which generate heat repeatedly by themselves are very large in difference in the coefficient of thermal expansion from the FR-4 printed wiring board of which coefficient of thermal expansion is about 13 to 20 ppm.
Accordingly, due to thermal stress after mounting parts, repeated stresses by expansion and contraction are concentrate on the electrodes for electrically connecting the substrate and the parts by soldering, and fatigue fracture may occur in the junction to cause breakage of wires.
The invention is devised in the light of such problems, and it is hence an object thereof to present a printed wiring board for surface mounting excellent in the connection reliability with the parts to be mounted on the surface.
To achieve the above-noted objects, the present invention adopts the following base technical constitution.
The first aspect of the present invention is a printed wiring board comprising: a printed wiring substrate having a plurality of a wiring layer, and a thermal expansion buffering sheet having a lower coefficient of thermal expansion than that of the printed wiring substrate, which is integrally laminated on a surface of the printed wiring substrate.
According to the first aspect of the present invention, since the printed wiring substrate is laminated with a thermal expansion buffering sheet of lower coefficient of thermal expansion than that of the printed wiring board, miss-matching of coefficient of thermal expansion between the surface mount parts and the printed wiring substrate is canceled by the thermal expansion buffering sheet, so that the reliability of mechanical and electrical connection of mounted parts is enhanced.
The second aspect of the present invention is characterized in that the coefficient of thermal expansion of the printed wiring substrate is 13 to 20 ppm, and the coefficient of thermal expansion of the thermal expansion buffering sheet is 6 to 12 ppm.
According to the second aspect of the present invention, miss-matching of coefficient of thermal expansion between the surface mount parts and the printed wiring substrate is canceled by the thermal expansion buffering sheet having lower coefficient of thermal expansion than that of the printed wiring substrate, so that the reliability of mechanical and electrical connection of mounted parts is enhanced.
The third aspect of the present invention is characterized in that the printed wiring substrate is a multi-layer wiring board which laminates wiring layers and insulation layers which are made of a glass cloth impregnated with an epoxy resin, alternately.
According to the third aspect of the present invention, it is possible to manufacture the printed wiring board at low cost by using the generally employed printed wiring substrate.
The fourth aspect of the present invention is characterized in that the thermal expansion buffering sheet is made of an aramid material.
According to the fourth aspect of the present invention, the aramid material has a coefficient of thermal expansion of about 9 ppm, being close to the coefficient of thermal expansion of the parts to be mounted, so that miss-matching of coefficient of thermal expansion between the surface mount parts and the printed wiring substrate is canceled, as the result, the reliability of mechanical and electrical connection of mounted parts is enhanced.
The fifth aspect of the present invention is characterized in that an electrode pattern so as to connect a part to be mounted on a surface of the printed wiring board is provided on a surface of the thermal expansion buffering sheet.
The sixth aspect of the present invention is characterized in that the part to be mounted on the surface of the printed wiring board is connected to the electrode pattern with a solder ball.